Advanced devices using a silicon wafers with a large diameter represented by, for example, a diameter of 300 mm are required to reduce surface waviness components, which are called nanotopography. Nanotopography is a kind of a surface shape of a wafer and exhibits irregularities of a wavelength component of 0.2 to 20 mm, which is shorter than the wavelength of a warpage or warp and longer than the wavelength of surface roughness. The nanotopography has an extremely shallow waviness component with a peak-to-valley value of 0.1 to 0.2 μm. It is said that the nanotopography affects yields of shallow trench isolation (STI) processes in device processes, and strict standards of nanotopography, together with the shrinking of design rules, are required of silicon wafers for use in device substrates.
Nanotopography is formed during processing of silicon wafers. The nanotopography is easy to degrade particularly in processing operations without a reference plane such as slicing with a wire saw or double-disc grinding. It is important to improve and manage relative meandering of a wire during slicing with a wire saw and wafer strain by double-disc grinding.
A conventional double-disc grinding method will now be described. FIG. 10 is a schematic diagram of an example of a conventional double-disc grinding apparatus.
As shown in FIG. 10, the double-disc grinding apparatus 101 includes a rotatable ring holder 102 configured to support a sheet workpiece W, a pair of hydrostatic supports 103 for supporting the ring holder 102 without contact by hydrostatic pressure of fluid, a pair of grinding wheels 104 for simultaneously grinding both surfaces of the workpiece W supported by the ring holder 102. The pair of hydrostatic supports 103 are located on the respective sides of the side faces of the ring holder 102. The grinding wheels 104 are attached to motors 112 and capable of rotating at a high speed.
With the double-disc grinding apparatus 101, the workpiece W is first supported along a circumferential direction from the outer circumference side of the workpiece by the ring holder 102. While the ring holder 102 is then rotated to rotate the workpiece W, fluid is supplied to spaces between the ring holder 102 and each of the hydrostatic supports 103 to support the ring holder 102 by the hydrostatic pressure of the fluid. In this way, both surfaces of the rotating workpiece W that is supported by the ring holder 102 and the hydrostatic supports 103 are ground with the grinding wheels 104 that are rotated at a high speed by the motors 112.
In conventional double-disc grinding, there are many factors that degrade nanotopography. As disclosed, for example, in Patent Document 1, it is known that a positional deviation of the ring holder along the direction of its rotational axis is one major factor. In view of this, it is known that a preferable supporting method to rotate a ring holder with high precision is to use a hydrostatic bearing for supporting the ring holder without contact by supplying fluid from both of the direction of the rotational axis of the ring holder and the direction perpendicular to the rotational axis (See Patent Document 2).
There is, however, a problem in that even when such a hydrostatic bearing is used, the nanotopography may degrade and thus highly precise nanotopography cannot be obtained stably.